The present invention relates to linear position measurement and in particular to linear position encoders.
Linear position encoders are known for measuring the position of a movable element or device along a travel path. One class of linear encoders are electro-optical in which a scale disposed along a travel path contains an array of optically sensible markings which are cooperative with markings on a member movable along the scale. Photosensors associated with the movable member provide electrical signals representative of movement with respect to the scale and provide an output representative of relative position of the movable member in relation to the fixed scale. Such electro-optical encoders can provide accurate linear measurement but are subject to contamination by dirt, dust or debris within the operating environment. Linear encoders are often used in machine tools or in other factory environments in which dust and dirt can be present.
Another class of linear encoders is based on inductive electrical principles. An elongated scale contains an electrical pattern in which a current is caused to flow. A movable member movable along the length of the scale contains an inductive pickup into which current is induced and which is sensed to provide an indication of relative movement of the member in relation to the scale. Such an inductive encoder is subject to electrical interference and is also subject to magnetic and electromagnetic interference which may be caused by the presence of permanent magnets, stray electromagnetic fields and the like.